C.C.I.R. (International Radio Consultative Committee) Recommendation No. 450, Stereophonic System for VHF Frequency Modulation Broadcasting, Oslo 1946, provides for a modulated 38 KHz subcarrier to provide the stereophonic information. To this end, a pilot signal of 19 KHz (half the sub-carrier frequency) is also transmitted. The carrier frequency of the transmitter (in the range 88-108 MHz) is modulated, inter alia, by this pilot signal.
One method of regenerating the sub-carrier frequency in a stereo receiver is by means of an oscillator locked to the pilot frequency. If the regenerated sub-carrier is not adjusted to the correct phase, channel separation is lost.
The digital phase locked loop circuit shown in FIG. 1 is schematic of a known circuit for extracting the pilot tone, prior to regeneration of the sub-carrier, from a received and demodulated FM transmitted stereo signal. The received pilot tone signal is in analog form and inevitably is noisy. It is not used directly. Instead, a reproduction thereof is synthesised using an oscillator which, if digital, is followed by a sinewave generator.
In FIG. 1 of the accompanying drawings, the input signal is sampled in an Analog to Digital converter (A/D converter) 1, the converter being clocked at a sampling frequency F.sub.s. A Numerically Controlled Oscillator (NCO) 4 is also clocked at the same frequency F.sub.s to provide an output which is fed to a sinewave generator 5. The output of the sinewave generator 5 provides the "extracted" pilot tone for subsequent use in a stereo receiver (not shown). A loop is provided by also feeding the output of the sinewave generator 5 to a mixer or multiplier 2 wherein it is mixed with the sampled input signal from the A/D converter 1.
An output of the mixer 2 is fed to a low pass filter 3 which passes the difference value to provide a correction signal. The correction signal is summed with a constant input value K in a summer 6 and the output of the summer 6 constitutes the input to the NCO 4, thus causing slight variations in the phase or output frequency of the NCO 4.
For accurate control of the frequency and phase of the output signal of the sinewave generator 5 (that is, the extracted pilot tone) and to ensure that it is locked to the frequency and phase of the input signal, the sinewave generator 5 must store a large number of values (usually in ROM from where they are extracted using look-up tables) to produce the requisite digitally represented sinewave output. The size of ROM and the range of addresses in a conventional sinewave generator is, in consequence, very large. Further, as the NCO 4 is clocked at the same rate, F.sub.s, as the sampling rate, the sinewave generator 5 must respond very rapidly.
It is an object of the present invention to provide an all-digital phase locked loop circuit incorporating a sinewave generator wherein the required address range of the generator is much reduced.